Two major wood preservatives, pentachlorophenol (PCP) and creosote, have been used to prepare wood for industrial applications. For example, PCP mixed with creosote and other hydrocarbons is used to treat utility poles as well as wooden packaging boxes as an herbicide, fungicide, pesticide and biocide. The average life of these utility poles is 20-35 years, after which they are discarded. Until now, wooden poles and railroad ties were disposed of in landfills or burned in power generation plants. However, due to reported carcinogenicity of PCP, in May of 1992 the Environmental Protection Agency proposed a substantial revision to their Hazardous Waste Identification System which would lower the hazardous waste level of PCP from 100 mg/kg dry wood to 0.1 mg/kg dry wood. If this level is adopted, disposal of PCP treated wood in landfills would be prohibited. In addition, the transportation cost and cost associated with complying with clean air requirements exceeds the BTU value of wood, thus making burning of this wood to generate power uneconomical. Therefore, environmentally safe methods of recycling these materials are greatly desired.
Many prior art approaches for removing creosote deposits utilize toxic petroleum distillates or other volatile agents which require great care in handling and application. Still other problems are associated with such compositions because they are generally non-biodegradable and often may not be completely removed without the introduction of another solvent.
U.S. Pat. No. 4,734,138 (Ely et al.) discloses methods for removing creosote deposits from wood using a biodegradable "wood restorative" composition consisting of water, sodium hydroxide, sodium bicarbonate, ethylene glycol, glycerol and corn starch. This composition can be applied to creosote-treated wood and allowed to dry and then removed by conventional cleaning methods. However, this method removes only surface deposits and would not render the wood recyclable.
U.S. Pat. No. 4,729,832 (Leonard et al.) teaches methods for reducing concentrations of phenol and naphthalene contaminants in a waste water stream recovered from a wood drying process where creosote is used. A cationic organic polyelectrolyte demulsifying agent and a coal tar based extractant are used in this process. The extractant phase is then returned to the wood preserving process for further use.
U.S. Pat. No. 5,262,004 (Gilbert et al.) discloses a method for extracting chemical preservatives from wood impregnated with PCP and/or creosote. The wood is shredded to produce wood chips which are then impregnated with an aqueous solution containing an alkali hydroxide. The chips are then crushed until most of the chemical preservatives are removed. The remaining wood particles and aqueous phase which contains the chemical preservatives are discarded.
Portier et al., "Recovery of Wood Fiber from Treated Wood Products by Combined Physical Chemical and Biological Approaches: Field Pilot Studies", Microterra, Inc., Boca Raton, Fla. (1993) describe a process for the recovery of wood fiber from products which have been chemically treated with PCP or creosote. In this method both the wood fiber and wood preservative can be recycled. In an initial chipping phase, the wood is shredded and splintered to increase the wood surface area prior to chemical pretreatment. The chips are then introduced into a contact reactor where alcohol extraction occurs for about fifteen minutes. The resulting alcohol/creosote and/or PCP liquor is then distilled and the distillate recycled. The treated wood chips are then inoculated with flavobacterium and acinetobacter species to polish any residual PCP and/or creosote from the wood. The treatment results in approximately 98.7% of the PCP being removed after 21 days at an approximate cost of $160.00/ton. Others have reported that the process takes three months and costs approximately $200/ton. Such long treatment makes this process undesirable for commercial use. In addition, the PCP content in utility poles has been found to be as high as 20,000 mg/kg of wood. Thus, remaining PCP in wood following treatment with this process is around 325 mg/kg of wood rendering the wood unusable for reuse in fiberboard or in paper manufacture. Careful control of environmental variables such as bacterial seeding, pH, oxygen concentration, aeration and temperature during the polishing step has been reported to increase mineralization of creosote and PCP to greater than 99.99% using this process.
There remains a need for rapid and efficient methods and systems for the removal of chemical preservatives such as PCP, creosote and/or mixtures thereof from wood products. There also remains a need for improved methods and systems which render treated wood products recyclable and useful. Accordingly, the method and system of the present invention, which removes virtually all of the chemical preservatives and allows both the wood and the preservative to be recycled, is highly desirable.